Cold storage material, refrigerator, device incorporating superconducting coil, and method of manufacturing cold storage material

ABSTRACT

A cold storage material, which has a large specific heat and a small magnetization in an extremely low temperature region and has satisfactory manufacturability, is provided, and a method for manufacturing the same is provided. Further, a refrigerator having high efficiency and excellent cooling performance is provided by filling this refrigerator with the above-described cold storage material. Moreover, a device incorporating a superconducting coil capable of reducing influence of magnetic noise derived from a cold storage material is provided. The cold storage material of embodiments is a granular body composed of an intermetallic compound in which the ThCr 2 Si 2 -type structure  11  occupies 80% by volume or more, and has a crystallite size of 70 nm or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of No. PCT/JP2019/037995, filed on Sep. 26, 2019, and the PCT application is based upon and claims the benefit of priority from Japanese Patent Application No. 2018-185628, filed on Sep. 28, 2018, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

Embodiments relate to a cold storage material to be used at an extremely low temperature and a technique to which this cold storage material is applied.

BACKGROUND

A superconducting electromagnet is used in a magnetic resonance imaging (MRI) system, a heavy particle beam accelerator which are operated in an extremely low temperature environment of several tens of K or less. Usually, this extremely low temperature environment is realized by a cold storage type refrigerator represented by a Gifford-McMahon (GM) refrigerator.

Several types of cold storage materials, which have a large specific heat for each operating temperature region, are used in the refrigerators. In the GM refrigerators that are currently widely used, Cu mesh is used as the cold storage material for the first cold storage device, and spherical particles of Pb and Bi alloy are used as the cold storage material on the high-temperature side of the second cold storage device, and particles of rare earth compounds such as Gd₂O₂S (GOS), HoCu₂, and Er₃Ni are used as the cold storage material on the low-temperature side of the second cold storage device. Among such cold storage materials, GOS has a high specific heat characteristic in a temperature region near 5K.

In order to synthesize an oxide cold storage material such as GOS, a multi-step process such as synthesis of raw materials, granulation, sintering at a high temperature, and spherical finishing by polishing is required.

Many refrigerators configured to achieve extremely low temperatures are used to cool superconducting coils. Thus, when the magnetization of the cold storage material is large, the cold storage material receives a large force due to the magnetic field generated by the superconducting coil and problems such as damage to the shaft containing the cold storage material may occur, which reduces the reliability of the refrigerator. Although superconducting coils are used for MRI and the like as described above, when the magnetization of the cold storage material is large, noise may be included in images due to magnetic noise derived from the cold storage material. Hence, the magnetization of the cold storage material is required to be small.

In refrigerators such as a GM refrigerator, a pulse tube refrigerator, and a Stirling refrigerator, high-pressure working gas fluidly reciprocates through the gap between the cold storage materials packed in the cold storage device. Further, in a GM refrigerator and a Stirling refrigerator, the cold storage device filled with cold storage materials vibrates. Thus, the cold storage materials are required to have mechanical strength.

In terms of manufacturing cold storage materials, it is preferred to use intermetallic compounds that can be manufactured by a simple process of melt-solidification. An oxide cold storage material such as GOS require a multi-step manufacturing process such as synthesis of raw materials, granulation, sintering at a high temperature, and spherical finishing by polishing. RCu₂X₂ (R=Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, X=Si, Ge) is known to have a large specific heat at extremely low temperatures as a choice for the cold storage material of intermetallic compounds.

However, the RCu₂X₇ compound is produced by, for example, melting the raw materials under an arc melting method and then subjecting the obtained ingot to a uniform heat treatment at a high temperature for a long time (for example, at 800° C. for one week). When a high-temperature and long-time heat treatment process is required after melt-solidification as described above, its cost increases in the case of being applied to industrial mass production.

PRIOR ART DOCUMENT Patent Document

-   [Patent Document 1] JP H09-014774 A -   [Patent Document 2] JP H06-101915 A

Non-Patent Document

-   [Non-Patent Document 1] L. Gonedek, et. al., Acta Phys Pol A 122,     391 (2012). -   [Non-Patent Document 1] Y. Takeda, et. al., J. Phys. Soc. Jpn. 77,     104710 (2008).

SUMMARY Problems to be Solved by Invention

A cold storage material, which has a large specific heat and a small magnetization in an extremely low temperature region and has satisfactory manufacturability, is provided, and a method for manufacturing the same is provided. Further, a refrigerator having high efficiency and excellent cooling performance is provided by filling this refrigerator with the above-described cold storage material. Moreover, a device incorporating a superconducting coil capable of reducing influence of magnetic noise derived from a cold storage material is provided.

Solution to Problem

The cold storage material of embodiments is a granular body composed of an intermetallic compound in which a ThCr₂Si₂-type structure compound occupies 80% by volume or more, and has a crystallite size of 70 nm or less.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a model diagram of a ThCr₂Si₂-type structure showing a crystal structure of a cold storage material according to the first embodiment.

FIG. 2 is a schematic diagram showing a grain shape of the cold storage material according to the first embodiment.

FIG. 3 is a cross-sectional view of a two-stage expansion type GM refrigerator exemplified as the refrigerator according to the second embodiment.

FIG. 4 is a cross-sectional view of an MRI apparatus exemplified as a device incorporating a superconducting coil according to the third embodiment.

FIG. 5 is a graph showing measurement results by a powder X-ray diffraction method for Example 1 in the upper part and for Comparative Example 1 in the lower part.

FIG. 6 is a graph showing the specific heat characteristics of Example 1 and Comparative Example 1 in an extremely low temperature region.

FIG. 7 is a graph showing measurement results by the powder X-ray diffraction method for Example 1 in the upper part and for Comparative Example 2 in the lower part.

FIG. 8 is a table showing crystallite size, volume % of the ThCr₂Si₂-type structure, proportion of each pulverized sample, peak temperature of specific heat, and peak values of specific heat for intermetallic compounds of DyCu₂Ge₂, DyCu₂Si₂, GdCu₂Si₂, PrCu₂Si₂, and TbCu₂Si₂ in Example 1 to Example 7 and Comparative Example 1 to Comparative Example 14.

FIG. 9 is a graph showing magnetization characteristics in an extremely low temperature region of Example 1.

DETAILED DESCRIPTION First Embodiment

Hereinafter, embodiments will be described in detail. FIG. 1 is a model diagram of a ThCr₂Si₂-type structure 11 showing the crystal structure of the cold storage material according to the first embodiment. The cold storage material according to the first embodiment is a granular body composed of an intermetallic compound in which ThCr₂Si₂-type structure 11 occupies 80% by volume or more, and has a crystallite size of 70 nm or less.

In the above-described ThCr₂Si₂-type structure 11, Th site 12 is at least one element selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y. In the above-described ThCr₂Si₂-type structure 11, Si site 14 is at least one element selected from Si and Ge, and Cr site 13 is at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt.

In a refrigerator such as a GM refrigerator described below, a working gas such as He gas fluidly reciprocates through the gap of the cold storage materials which fill the cold storage device, the low temperature generated by the compression/expansion cycle of the gas is stored in the cold storage material, and thereby, the refrigerator cools down from a room temperature to an extremely low temperature. Thus, the cold storage material to be mounted on the refrigerator is required to have a large specific heat characteristic in the operating temperature region.

When the ThCr₂Si₂-type structure 11 occupies 80% by volume or more in this intermetallic compound, the intermetallic a cold storage material having a high specific heat characteristic in an extremely low temperature region can be obtained. If the proportion of the ThCr₂Si₂-type structure 11 in the intermetallic compound is less than 80% by volume, the specific heat characteristic is deteriorated in some cases compared with general substances listed as cold storage materials in the extremely low temperature region. The volume % of the ThCr₂Si₂-type structure can be calculated from the Rietveld analysis of the powder X-ray diffraction method and/or evaluation of the ratio of the phases of a plurality of fields of view by observation with a scanning electron microscope.

In a GM refrigerator and a Stirling refrigerator, the cold storage device filled with the cold storage material vibrates, and thus, the cold storage material is required to have mechanical strength. Thus, crystallite size of the cold storage material is as fine as 70 nm or less, which ensures excellent mechanical strength of the cold storage material. The crystallite size L is calculated by evaluating the peak width (half width) β in the X-ray diffraction pattern and using Scherrer's equation (Equation (1)). When the crystallite size is small, the half width of the X-ray diffraction pattern becomes large.

L=Kλ/(β cos θ)  (1)

(wherein K is the Scherrer constant and A is wavelength of X-rays to be used) The mechanical strength can be evaluated by a vibration test.

If the crystallite size of the cold storage material is larger than 70 nm, the mechanical strength is reduced, the granules are fatigue-fractured and pulverized as the period of use elapses, and the predetermined performance of the refrigerator cannot be maintained. The crystallite size is preferably 1 nm or more, more preferably 10 nm or more.

FIG. 2 is a schematic diagram showing a grain shape of the cold storage material according to the first embodiment. As the particle size of the granular body of the cold storage material, φmax is defined as the length of the granular body in the longest direction and φmin is defined as the length of the longest portion in the direction perpendicular to the longest direction. φmax and φmin are included in the range of 0.01 mm to 1 mm, and more preferably, φmax and φmin are included in the range of 0.05 mm to 0.5 mm. When an area of a projected image 15 of this cold storage material is defined as A and an area of the smallest circumscribed circle 16 circumscribing this projected image 15 is defined as M, the shape coefficient represented by M/A is included in the range of 1.0 to 5.0 in every projection direction.

Since the particle size of the cold storage material is included in the range of 0.01 mm to 1 mm, in the refrigerator described below, the flow of the working gas (He gas) that fluidly reciprocates in the cold storage device filled with the cold storage material is not obstructed, and satisfactory heat exchange between the working gas and the cold storage material is achieved. When the particle size of the cold storage material is less than 0.01 mm (10 μm), the gap between the particles of the cold storage material (i.e., space through which the working gas flows) may be narrowed and the pressure loss of the gas may increase. When the particle size of the cold storage material is larger than 1 mm, the filling rate of the cold storage material in the cold storage device may decrease and the heat exchange between the working gas and the cold storage material may decrease.

The production of such a cold storage material is performed by at least going through a process of blending and melting the component elements of an intermetallic compound, which can form the above-described ThCr₂Si₂-type structure 11, at its stoichiometric ratio and a process of injecting this molten liquid into a dynamic cooling medium and rapidly cooling and solidifying it into granules.

That is, the elemental metal blended to have the stoichiometric ratio of the ThCr₂Si₂-type structure 11 is melted by high frequency induction heating or the like. Further, the molten metal is supplied to the surface of a high-speed rotating body installed in the atmosphere of vacuum or inert gas. This molten metal is finely dispersed by the motion of the rotating body and is simultaneously subjected to rapid-solidification so as to form spherical granules. Additionally or alternatively, the above-described molten metal is made to flow out into a vacuum or an atmosphere of inert gas, and a non-oxidizing atomizing gas is allowed to act on it. As a result, the molten metal is atomized and dispersed, and at the same time, is rapidly cooled and solidified to form spherical granules.

Specific methods for rapidly cooling and solidifying the above-described molten metal include a rotary disc process (RDP) method, a single roll method, a double roll method, an inert gas atomizing method, and a rotary nozzle method. According to these methods, the molten metal can be rapidly cooled at a cooling rate of 10⁵ to 10⁶° C./s, and thus, an intermetallic compound having the ThCr₂Si₂-type structure can be readily produced in the form of granules at low cost. Details of the rapid-solidification method for the molten metal are described in Japanese Patent No. 2609747.

When an intermetallic compound having a different magnetic-phase transition-temperature is added to the intermetallic compound having the ThCr₂Si₂-type structure, the specific heat characteristic per unit volume of the cold storage material can be enhanced. For example, when a phase having an AlB₂-type structure and a LiGaGe-type structure is present in the intermetallic compound having the ThCr₂Si₂-type structure, the specific heat in the 4K to 20K region can be increased. When a phase having a Gd₃Cu₄Ge₄-type structure is present in the intermetallic compound having the ThCr₂Si₂-type structure, the specific heat in the vicinity of 7K to 50K can be increased. However, when the amount of the phases excluding the ThCr₂Si₂-type structure is 20% by volume or more, the volume specific heat derived from the ThCr₂Si₂-type structure becomes small. When the intermetallic compound is composed of phases having different crystal structures, the mechanical strength of the cold storage material can be increased.

Second Embodiment

FIG. 3 is a cross-sectional view of a two-stage expansion type GM refrigerator exemplified as the refrigerator 30 according to the second embodiment. The refrigerator 30 includes: a large-diameter first cylinder 31; and a small-diameter second cylinder 32 that is coaxially connected to the first cylinder 31. A first cold storage device 34 is disposed in the first cylinder 31 so as to be able to reciprocate, and a second cold storage device 35 is disposed in the second cylinder 32 so as to be able to reciprocate. A seal ring 36 is disposed between the first cylinder 31 and the first cold storage device 34, and a seal ring 37 is disposed between the second cylinder 32 and the second cold storage device 35.

A first expansion chamber 41 is provided between the inner wall of the first cylinder 31 and the connection portion of the first and second cold storage devices 34 and 35. A second expansion chamber 42 is provided between the second cold storage device 35 and the tip wall of the second cylinder 32. A first cooling stage 43 is formed at the bottom of the first expansion chamber 41, and a second cooling stage 44 being lower in temperature than the first cooling stage 43 is formed at the bottom of the second expansion chamber 42.

In the first cold storage device 34, a first cold storage material 38 such as a copper alloy mesh is accommodated under the state where a passage 33 of the working gas (He gas, and the like) is secured. As the first cold storage material 38, a stainless steel mesh may be used instead of the copper alloy mesh, and both of them may be used. The second cold storage device 35 is filled with second cold storage materials 40 in a form in which the passage 39 of the working gas is secured. Although a description has been given of the cold storage devices 34 and 35 in which the first cold storage material 38 and the second cold storage materials 40 are packed separately, these may be packed in one cold storage device.

The second cold storage materials 40 to be housed inside the second cold storage device 35 are filled with a plurality of types of second cold storage materials 40 a and 40 b such that these second cold storage materials 40 a and 40 b are partitioned by a mesh 48. The filling rate of the second cold storage materials 40 a and 40 b in the space partitioned by the mesh 48 is preferably 50% to 75% in consideration of the fluidity of the working gas, and is more preferably 55% to 65%.

In the two-stage refrigerator 30, the working gas (He gas or the like) is compressed by a compressor 45 and supplied to the refrigerator 30 through a high-pressure line 46. The supplied working gas passes through the gap of the first cold storage material 38 housed in the first cold storage device 34 so as to reach the first expansion chamber 41, and then cools the first cooling stage 43 by expansion. Next, the working gas passes through the gap of the second cold storage materials 40 housed in the second cold storage device 35 so as to reach the second expansion chamber 42, and then cools the second cooling stage 44 by expansion.

The working gas having been made into low-pressure state passes through the second cold storage device 35 and the first cold storage device 34 in this order (i.e., in the order opposite to the case of the high-pressure), and then is returned to the compressor 45 through a low-pressure line 47. Afterward, it is compressed by the compressor 45 and the above-described cycle is repeated. The expansion of each of the expansion chambers 41 and 42 is realized by the reciprocating operation of the cold storage devices 34 and 35. At this time, each of the cold storage materials 38 and 40 exchanges heat energy with the working gas so as to accumulate and retain cold heat, and also performs heat regeneration.

Next, the cycle will be described focusing on the heat flow. The high-pressure working gas to be supplied from the compressor 45 to the refrigerator 30 is at room temperature (about 300K). This working gas is precooled by the first cold storage material 38 while passing through the first cold storage device 34, and then reaches the first expansion chamber 41. Afterward, the working gas expands in the first expansion chamber 41 so as to be further lowered in temperature and thereby cools the first cooling stage 43. Subsequently, the working gas is precooled by the second cold storage materials 40 while passing through the second cold storage device 35, and then reaches the second expansion chamber 42. Afterward, the working gas expands in the second expansion chamber 42 so as to be further lowered in temperature and thereby cools the second cooling stage 44.

The working gas having been made into the low-pressure state passes through the inside of the second cold storage device 35 while storing cold heat in the second cold storage materials 40 (i.e., while the working gas itself is being warmed). Subsequently, the working gas passes through the inside of the first cold storage device 34 so as to be warmed to near room temperature while storing cold heat in the first cold storage material 38 (i.e., while the working gas itself is being warmed), and then passes through the low-pressure line 47 so as to return to the compressor 45.

During the steady operation of the refrigeration cycle, a temperature gradient occurs in the cold storage materials 38 and 40 inside the cold storage devices 34 and 35. In such a refrigeration cycle, the larger the specific heat of the cold storage material at the operating temperature is, the higher the thermal efficiency of the working gas cycle becomes, which achieves lower temperature and higher refrigeration performance.

In general, specific heat of a solid has the property of changing depending on the temperature. Thus, in particular, in order to enhance the heat recovery effect of the second cold storage materials 40, it is effective to selectively dispose the second cold storage materials 40 having satisfactory heat recovery characteristics in the respective temperature regions depending on the temperature gradient. Hence, the second cold storage device 35 is filled with a plurality of second cold storage materials 40 (40 a, 40 b) having different heat recovery characteristics.

In order to obtain a satisfactory heat recovery effect, the following characteristic are important. That is, the heat capacity (specific heat) of the cold storage material at the operating temperature of each portion in the cycle process is large, and the heat exchange between the cold storage materials 40 and 38 and the working gas is satisfactory. In the first cold storage device 34, the temperature region from room temperature to 100K or less is the main operating temperature region, and thus, Cu having a large specific heat per unit volume in this temperature region is selected. Further, Cu mesh is widely used as the first cold storage material 38 because mesh subjected to wire-drawing process is industrially easy to use.

Pb and Bi, which have a higher specific heat at 60K or less than Cu, are selected as the second cold storage material 40 a on the high-temperature side of the second cold storage device 35. The cold storage material having the ThCr₂Si₂-type structure according to the first embodiment, which has a higher specific heat at 8K or less than Pb and Bi, is selected as the second cold storage material 40 b on the low-temperature side of the second cold storage device 35. In consideration of the temperature gradient inside the cold storage devices 34 and 35, for the cold storage materials 38 and 40 of the GM refrigerator, it is preferred to select and dispose substances having a large volume specific heat in the operating temperature region of each portion in the above-described manner. Note that the second cold storage material 40 a to be disposed on the high-temperature side of the second cold storage device 35 is not limited to Pb and Bi. HoCu₂, Er₃Ni, and the like may be disposed as the second cold storage material 40 a. Additionally, the second cold storage materials 40 are not limited to the above-descried two layers but may be formed of three or more layers.

Further, the refrigerator equipped with the cold storage material according to the first embodiment is not limited to the above-described GM refrigerator. In refrigerators configured to generate an extremely low temperature from a room temperature, such as a pulse tube refrigerator, a Claude refrigerator, and a Stirling refrigerator, the cold storage materials are disposed at portions where a large thermal impedance is required, such as the boundary region between the cold and hot portions to be caused by the compression/expansion cycle of the working gas.

Third Embodiment

FIG. 4 is a cross-sectional view of a magnetic resonance imaging (MRI) apparatus 50 exemplified as a device incorporating a superconducting coil according to the third embodiment. In the diagnosis by this MRI apparatus 50, a movable base (not shown) on which a subject 52 lies is moved into a tunnel-shaped bore space 51. Further, a static magnetic field is applied by a first electromagnet 53, and a gradient magnetic field is applied by the second electromagnet 54.

Further, a radio wave is transmitted from an RF coil 55, and a magnetic resonance signal is received as a response signal from the subject 52. Due to the gradient magnetic field, positional information on the position where the response signal is generated is also received at the same time. The received response signals are analyzed by a signal processing system (not shown) to reconstruct an internal image of the body of the subject 52.

In the MRI apparatus 50 currently used as the mainstream, a superconducting coil configured to generates a strong magnetic field such as 1.5 T and 3 T is used for the first electromagnet 53. The stronger the magnetic field is, the better the S/N (signal/noise) ratio of the magnetic resonance response signal becomes, which enables generation of a clearer image. As the superconducting coil to be used for the first electromagnet 53, a solenoid coil wound with metal-type low-temperature superconducting wires such as NbTi and Nb₃Sn is usually used.

Since these wires need to be kept at the critical temperature of the superconducting transition or lower, the first electromagnet 53 is installed in a He bath 56 filled with liquid He that liquefies at 4.2K (about −269° C.) or less under 1 atm. Since the liquid He is rare and expensive, an adiabatic vacuum layer 57 is provided on the outside of the He bath 56 in order to suppress evaporation of the liquid He. Further, in order to reduce the influence of heat intrusion from the environment (room temperature: about 300K) in which the MRI apparatus 50 is installed, two radiation shields 58 and 59 are provided in the adiabatic vacuum layer 57. The shield 58 is cooled to about 4K and the shield 59 is cooled to about 40K by the installed refrigerator 30.

The refrigerator 30 is not limited to a particular one, and a GM refrigerator and a JT refrigerator may be used in combination as the refrigerator 30. Additionally or alternatively, a refrigerator such as a GM refrigerator, a pulse tube refrigerator, a Claude refrigerator, and a Sterling refrigerator is used alone as the refrigerator 30 in some cases. In particular, the GM refrigerator has significantly improved in refrigeration performance by being equipped with a magnetic cold storage material in the 1990s, which has enabled generation of an extremely low temperatures below the liquid He temperature by using only the GM refrigerator. Thus, the GM refrigerator is often used in the MRI apparatus 50, which is widely used at the time of filing of the present application.

As shown in FIG. 4 , the first cooling stage 43 (FIG. 3 ) of the GM refrigerator 30 is connected to the shield 59, and the second cooling stage 44 (FIG. 3 ) is connected to the shield 58. At the time of filing of the present application, GM refrigerators capable of stably obtaining a refrigeration capacity of 1 W or more at 4K are widespread. Thus, the heat invasion into the He bath 56 and the cooling by the GM refrigerator 30 are balanced, and thereby, an extremely low temperature can be maintained and evaporation of the liquid He can be almost completely suppressed.

Consequently, in medical institutions such as hospitals, when the liquid He is injected at the initial start-up of the MRI apparatus 50, it is not necessary to regularly add the liquid He, which is expensive and not easy to handle, in the subsequent operation. Due to this significant improvement in convenience, the introduction of MRI apparatus 50 to small and medium-sized hospitals is expanding. In addition, an MRI apparatus including a direct cooling type superconducting coil, which conducts and cools the superconducting coil with a refrigerator without using the liquid He, has also been commercialized. In the case of such an MRI apparatus, the liquid He bath 6 can be omitted.

In recent years, MRI apparatuses using high-temperature superconducting wires such as Y-type, Bi-type, and MgB₂ have been developed. In also these apparatuses similarly to the MRI apparatus using a low-temperature superconducting material, the superconducting coil needs to be equal to or lower than the critical temperature of the superconducting transition and needs to be cooled below 10K to 30K (about −257° C.) at which the current required to generate the magnetic field can flow.

Thus, in the MRI apparatus using a high-temperature superconducting material, it is necessary to cool the superconducting coil by applying conduction cooling on the superconducting coil with the use of a refrigerator or by submerging it in liquid He, liquid H₂, and/or liquid Ne, liquefaction temperature of each of which is 4K to 30K (about −269° C.) or lower under 1 atm. Also in the latter method, it is desirable to cool it by using a refrigerator in order to prevent evaporation of liquids He, liquid H₂, and liquid Ne. In order to improve the performance of the refrigerator at 10K to 30K, it is preferred to mount a cold storage material having a large specific heat in the same temperature region on the refrigerator.

The device incorporating a superconducting coil (i.e., an apparatus with a built-in superconducting-coil) according to the third embodiment is equipped with the refrigerator of the second embodiment, which is provided with the cold storage material of the first embodiment. The magnetization of this cold storage material is 10 emu/g or less, more preferably 5 emu/g or less, and further preferably 2 emu/g or less at an external magnetic field of 1000 Oe and a temperature of 5K or lower. Since the magnetization of the cold storage material is small as described above, the influence of magnetic noise derived from the cold storage material can be reduced and a high-quality image can be obtained. The device incorporating a superconducting coil according to the third embodiment is not limited to the above-described MRI apparatus 50. The device incorporating a superconducting coil according to the third embodiment includes a superconducting magnet for a magnetically levitated train, a superconducting magnet device, a cryopump device, a Josephson voltage standard device, and a magnetic-field application type monocrystal pulling device.

In particular, the cryopump device achieves a high degree of vacuum by being cooled to about 10K. Thus, the performance of the cryopump device can be improved by mounting a cold storage material having a large specific heat in the vicinity of 10K on the refrigerator.

EXAMPLES Example 1, Comparative Example 1

Next, Example 1 will be described in more detail. The elemental metals, which are components of the intermetallic compound DyCu₂Ge₂, are used as the raw materials and are mixed in its stoichiometric ratio so as to be melted. Further, the distance between the nozzle and the roll is set to 0.5 mm under the roll quenching method, and a flaky sample was prepared by rapid-solidification at a cooling rate of 10⁵° C./sec to 10⁶° C./sec. As Comparative Example 1, a bulk sample was prepared by slowly cooling and solidifying the raw materials at a cooling rate of 10²° C./sec using the arc melting method under the same compounding and melting conditions as Example 1.

Example 2

A flaky sample was prepared under the same conditions as in Example 1 except that the distance between the nozzle and the roll was set to 0.6 mm.

Example 3

A flaky sample was prepared under the same conditions as in Example 1 except that the distance between the nozzle and the roll was set to 0.7 mm.

FIG. 5 is a graph showing measurement results by a powder X-ray diffraction method for Example 1 in the upper part and for Comparative Example 1 in the lower part. The measurement of the powder X-ray diffraction was performed by using SmartLab manufactured by Rigaku Co., Ltd. From the X-ray diffraction pattern of this graph, it can be seen that most of the crystal structure of the intermetallic compound of Example 1 obtained by the rapid-solidification treatment is DyCu₂Ge₂. It can be seen that the intermetallic compound of Comparative Example 1 obtained by the slow-cooling solidification treatment also contains a plurality of subphases.

FIG. 6 is a graph showing the specific heat characteristics of Example 1 and Comparative Example 1 in the extremely low temperature region. The specific heat characteristics were measured with the use of a physical property measurement system (PPMS) manufactured by Quantum Design Japan, Inc. As shown in FIG. 6 , it can be seen that Example 1 subjected to the rapid-solidification treatment is larger in local maximum value of the specific heat in the low temperature region than Comparative Example 1 subjected to the slow-cooling solidification treatment. As a result, the cooling capacity of the refrigerator is improved by adopting the intermetallic compound of Example 1 as the cold storage material to be packed in the cold storage device of the refrigerator.

Comparative Example 2

Under the same compounding and melting conditions as in Comparative Example 1, a bulk sample was prepared by heat treatment at 800° C. below the solidifying point for one week. The sample preparation conditions of Comparative Example 2 reproduce the above-described disclosure conditions of Non-Patent Document 1.

Comparative Example 3

A bulk sample was prepared under the same conditions as in Comparative Example 2 except that the heat treatment was performed at 900° C. below the solidifying point for four days.

Comparative Example 4

A bulk sample was prepared under the same conditions as in Comparative Example 2 except that the heat treatment was performed at 800° C. below the solidifying point for four days.

Comparative Example 5

A bulk sample was prepared under the same conditions as in Comparative Example 2 except that the heat treatment was performed at 700° C. below the solidifying point for four days.

Comparative Example 6

Under the same compounding conditions as in Example 1, a bulk sample was prepared by performing slow-cooling solidification at a cooling rate of 10²° C./sec under the high-frequency dissolution method.

Example 4

A flaky sample was prepared under the same conditions as in Example 1 except that the composition was DyCu₂Si₂.

Comparative Example 7

A bulk sample was prepared under the same conditions as in Comparative Example 1 except that the composition was DyCu₂Si₂.

Comparative Example 8

A bulk sample was prepared under the same conditions as in Comparative Example 7 except that the heat treatment was performed at 900° C. below the solidifying point for four days.

Example 5

A flaky sample was prepared under the same conditions as in Example 1 except that the composition was GdCu₂Si₂.

Comparative Example 9

A bulk sample was prepared under the same conditions as in Comparative Example 1 except that the composition was GdCu₂Si₂.

Comparative Example 10

A bulk sample was prepared under the same conditions as in Comparative Example 9 except that the heat treatment was performed at 900° C. below the solidifying point for four days.

Example 6

A flaky sample was prepared under the same conditions as in Example 1 except that the composition was PrCu₂Si₂.

Comparative Example 11

A bulk sample was prepared under the same conditions as in Comparative Example 1 except that the composition was PrCu₂Si₂.

Comparative Example 12

A bulk sample was prepared under the same conditions in Comparative Example 11 except that the heat treatment was performed at 900° C. below the solidifying point for four days.

Example 7

A flaky sample was prepared under the same conditions as in Example 1 except that the composition was NdCu₂Si₂.

Comparative Example 13

A bulk sample was prepared under the same conditions as in Comparative Example 1 except that the composition was NdCu₂Si₂.

Comparative Example 14

A bulk sample was prepared under the same conditions as in Comparative Example 13 except that the heat treatment was performed at 900° C. below the solidifying point for four days.

FIG. 7 is a graph showing measurement results by the powder X-ray diffraction method for Example 1 in the upper part and for Comparative Example 2 in the lower part. Example 1 shown in the upper part of FIG. 7 and Example 1 shown in the upper part of FIG. 5 are the same data except the scale display on the horizontal axis. As shown in FIG. 7 , it can be seen in Comparative Example 2 that the X-ray diffraction pattern of the unintended crystal structure being present in Comparative Example 1 disappears and most of the crystal structure becomes DyCu₂Ge₂ similarly to Example 1 by maintaining a high temperature and heat-treating the solid phase.

Comparing the X-ray diffraction patterns in FIG. 7 between Example 1 and Comparative Example 2, it can be seen that the peak spread is larger in Example 1. The peak identified as ThCr₂Si₂-type structure was used for calculating the crystallite size from the half width β. Even if the crystal structure of the intermetallic compound is the same, Example 1 subjected to rapid-solidification treatment is smaller in crystallite size than Comparative Example 2 subjected to heat-treatment at a high temperature in a solid phase, and has excellent mechanical properties.

The samples were put into a container (D=15 mm, h=14 mm) of a vibration tester, and a simple vibration with a maximum acceleration of 300 m/s² was applied 1×10⁶ times by the vibration tester. After this test, the mechanical strength of each sample was evaluated by appropriately classifying the samples in terms of shape and sieving the samples and by determining the weight ratio of each pulverized sample.

FIG. 8 is a table of results showing crystallite size, content percentage of ThCr₂Si₂-type structure, proportion of pulverized sample, peak temperature of specific heat, and peak value of specific heat in the samples of Example 1 to Example 7 and Comparative Example 1 to Comparative Example 14. When the crystallite size is larger than 70 nm, the proportion of pulverization is significantly increased and the mechanical strength is reduced. When the content percentage of the ThCr₂Si₂-type structure is less than 80% by volume, the peak value of specific heat is significantly reduced.

FIG. 9 is a graph showing the magnetization characteristics in the extremely low temperature region for Example 1. The magnetization characteristics were measured with the use of a magnetic property measurement system (MPMS) manufactured by Quantum Design Japan, Inc. When the external magnetic field is 1000 Oe, the magnetization in the temperature region 2K to 5K is 0.97 emu/g or lower. The magnetization of GOS, which has almost the same high specific heat characteristics as in Examples 1 to 3 in the temperature region near 5K, is 1.5 emu/g, the magnetization of HoCu₂ used on the low-temperature side of the second cold storage device excluding GOS is 3.5 emu/g, and the magnetization of Er₃Ni is 7 emu/g. Accordingly, the cold storage materials of Examples 1 to 3 have small magnetization characteristics, and thus contribute to improvement of image quality and reduction of magnetic noise of the device incorporating a superconducting coil when being mounted on an MRI apparatus.

In the case of the cold storage material of Example 1 having a granular particle size below 0.01 mm (10 μm), the gap between the particles of the cold storage material (i.e., space through which the working gas flows) becomes narrower, and the pressure loss of the gas increases, which deteriorates the refrigeration performance. When the granular particle size of the cold storage material is larger than 1 mm, the filling rate of the cold storage material in the cold storage device decreases, and thus the refrigeration performance deteriorates.

According to the cold storage material of at least one embodiment as described above, a cold storage material, which has a large specific heat and a small magnetization in an extremely low temperature region and has satisfactory manufacturability, can be provided. Additionally, a refrigerator having high efficiency and excellent cooling performance can be provided by filling the refrigerator with this cold storage material. Further, a device incorporating a superconducting coil capable of reducing the influence of magnetic noise derived from the cold storage material can be provided.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. These embodiments may be embodied in a variety of other forms, and various omissions, substitutions, and changes may be made without departing from the spirit of the inventions. These embodiments and their modifications are included in the accompanying claims and their equivalents as well as included in the scope and gist of the inventions.

REFERENCE SIGNS LIST

-   11 ThCr₂Si₂-type structure -   12 Th site -   13 Cr site -   14 Si site -   projected image -   16 circumscribed circle -   refrigerator -   31 first cylinder -   32 second cylinder -   33 passage of working gas -   34 first cold storage device -   second cold storage device -   36, 37 seal ring -   38 first cold storage material -   39 passage of working gas -   40 (40 a, 40 b) second cold storage materials -   41 first expansion chamber -   42 second expansion chamber -   43 first cooling stage -   44 second cooling stage -   compressor -   46 high-pressure line -   47 low-pressure line -   48 mesh -   50 MRI apparatus -   51 bore space -   52 subject -   53 first electromagnet -   54 second electromagnet -   55 RF coil -   56 He bath -   57 adiabatic vacuum layer -   58, 59 shield 

1-6. (canceled)
 7. A refrigerator provided with a cold storage material, the cold storage material being configured as a granular body composed of an intermetallic compound, wherein a ThCr₂Si₂-type structure occupies 80% by volume or more in the granular body; and a crystallite size of the granular body is 70 nm or less.
 8. A device incorporating a superconducting coil equipped with the refrigerator according to claim 7, wherein magnetization of the cold storage material is 10 emu/g or lower at an external magnetic field of 1000 Oe and a temperature of 5K or lower. 